1. Field of the Invention
The present invention relates to transport of messages specifying routing information between Internet Protocol (IP) routers according to a prescribed routing protocol, for example a distance vector routing protocol such as Enhanced Interior Gateway Routing Protocol (EIGRP).
2. Description of the Related Art
Wide area packet switched networks such as the Internet have become an integral part of worldwide commerce in part due to the ability of different networks to interoperate without central control. In particular, the decentralization of control is possible due to routing protocols which enable routers to communicate amongst each other and share routing information: routing protocols include operations such as router advertisement, router discovery, link state advertisement, and the sharing of all or at least a portion of respective routing tables. Distance-vector routing protocols call for each router to send all or a portion of its routing table in a routing-update message at regular intervals to each of its neighboring routers. Hence, each router can build a topology table that provides a detailed representation of the network topology relative to the corresponding router, and a routing table that enables routing of packets according to the network topology
One particular routing protocol of interest is disclosed in U.S. Pat. No. 5,519,704 to Farinacci et al., the disclosure of which is incorporated in its entirety herein by reference. Farinacci et al. describes a router configured for executing a distance vector routing protocol, referred to as Enhanced Interior Gateway Routing Protocol (EIGRP). As described in Farinacci et al., EIGRP enables routers to initially exchange routing information, including topology tables, enabling each router to identify its neighboring topology. Once the routers have established their respective topology tables and routing tables, a router only needs to send an EIGRP-based update message to another router only if a link transition occurs that affects the network topology.
EIGRP also utilizes the Diffusing Update Algorithm (DUAL), developed by J. J. Garcia-Luna-Aceves. DUAL enables EIGRP routers to determine whether a path advertised by a neighbor is looped or loop-free, and enables an EIGRP router to locate alternate paths without waiting for updates from other routers. Hence, upon receiving an EIGRP update message indicating a link transition, the EIGRP router updates its routing table and utilizes the DUAT algorithm to identify loop-free path, determine a most likely successor path based on cost metrics, and update its routing table accordingly. EIGRP routers are commercially available from Cisco Systems, San Jose, Calif.
The EIGRP protocol specifies that each link for a corresponding destination specified in a routing table of a given router can have one of two possible states, mainly “Active” and “Passive”. The “Active” state refers to a state when a link is not available (e.g., when a link failure occurs), at which point the router is “actively” attempting to identify from its topology a feasible successor to reach the destination. The “Passive” state refers to a state where the router is “passive” (i.e., does not need to identify a feasible successor) because the topology table already identifies an available link (i.e., a link that can reach the destination within prescribed cost parameters). Routers can generate update messages in response to: (1) receiving an update message from another router, and entering an “Active” phase to identify a feasible successor; or (2) detecting a transition in a link due to either a link failure (i.e., available link transitions to an unavailable link), or a link recovery (i.e., an unavailable link transitions to an available link).
Although existing EIGRP routers can determine network topology and generate routing tables accordingly based on link availability, the existing EIGRP protocol limits the generation of a new update message by a source router (i.e., a router not having received an update message related to the new update message) to only instances of detecting a link failure or a link recovery. The limiting of new update messages provides the benefit of minimizing routing protocol traffic between routers, but at the expense of limiting the exchange of useful information between routers.